Through the encounter with the bible salesman, what does Hulga learn about herself and other people?
Hulga learns that she is not as different from those around her as she wishes to be. Her education (a PhD in philosophy) has given her a sense of snobbery, and she feels above what she calls "good country people." Hence the title. She finds her mother and those around her simple, believing in the day-to-day duties of life, and holding onto religious faith. Hulga holds this faith system in contempt, even going so far as to change her given name (Joy) to the ugliest sounding name possible. It is her way of declaring she is above all these petty, provincial beliefs.
Yet her encounter with the traveling bible salesman changes this. by taking her leg, he renders her powerless, unable to command herself with her usual haughtiness. He shows her that the simpleness is a false front, covering depravity she could not know existed. She had gone with him because she wanted the power, wanted to control a simple country boy. But he turns the tables, responding to her educated nihilism by revealing his own, gained from a life of independence, rather than university courses.
This is the irony. Hulga wants him to be "good country people", because then she can feel above him. But his philosophy is just as sophisticated as her own, although it developed in a different manner. Thus she learns that she is not above everyone, and indeed, she has more in common with those around her than she previously thought.